Chrome packaged app - write javascript to Check license API if user has paid.

We have a packaged application for google chrome (written in HTML and JavaScript). This application is uploaded to chrome web store, and we want users to pay for the app. Currently the app downloads and runs for all users for free. If I understand it correctly, the app needs to call google's License API to find out if the user has paid, in order to work only for those users who paid. But the API reques some strange tokens, oauth token, access token, user open id or something, etc. So, your goal of this project is following: - you will guide us step by step in the procedure of obtaining all the tokens and IDs - you will write a short javascript function which will call the API to check if user has paid - if possible, when the user did not pay yet, he should be given the opportunity to pay right away - we will add this functionality to our packaged app and reupload it to chrome web store - when everything is done, the app should run only for those users who purchased the subscription and are paying monthly.